Recently there has been renewed interest in sustaining economic growth through utilization of fossil energy resources, such as coal, in an efficient and environmentally responsible manner. Advanced technology for power plants and gasifiers is important for the clean production of electric power, hydrogen generation, gasification of methane, production of industrial chemicals, refined fuels with reduced impact on water resources, solid waste disposal and capture of carbon dioxide generated in the use of fossil fuels. To meet the demand of future energy innovation, these industrial gases will be produced by clean processes. Sensors and controls are important ingredients in any modern process plant and the coal utilization plants will not be different. The development of effective sensors is vital, and such important technology must be available when needed so there is an effective process and rapid public acceptance of fossil energy utilization. Sensors will not only enable the clean, efficient, and low-cost process, but will also provide for safety in the workplace, home and environment. There is a need for sensors to be located as close to the process points as possible for control of the processes. In addition, there is a need for sensors at exit gas streams that feed auxiliary processes for clean-up or conditioning. Moreover, safety sensors in the plant, the surrounding environment and public spaces would help accelerate public acceptability and the pace of coal technology utilization. Safety sensors may also be utilized for medical and health reasons, especially sensors worn by an individual that may provide immediate feedback on the individual's health and safety. The requirements for such sensors typically exceed the capabilities of current sensors. Several major limitations for current process sensors include: potential severe conditions in and near process streams, interferences of the complex process stream components and the desired analytical measurement, slow response times for analytical information, the need for very low operational power requirements making the sensors incompatible with modern wireless systems, and especially the cost of deployment and ownership. Similarly, safety and environmental sensors are typically too costly and lack performance for easy, wide-spread deployment. So not only do sensors need to be cost effective for widespread deployment, but they also have to be simultaneously low power and tiny so they can be easily interfaced to process, safety, health, and environmental systems with and without wireless and other communication interfaces.
One recent innovation in the manufacturing of devices is microelectromechanical systems (MEMS) technology. MEMS technology is based on a number of tools and methodologies, which are used to form small structures with dimensions in the micrometer scale (one-millionth of a meter). Significant parts of the technology have been adopted from integrated circuit (IC) technology. For example, almost all devices are built on wafers of silicon like IC's. The structures are realized in thin films of materials and patterns using photolithographic methods. There are three basic building blocks in MEMS technology: 1) deposit thin films of material on a substrate, 2) apply a pattern mask on top of the films by photolithographic imaging, and 3) etching the film selectively in the mask. A MEMS process is usually a structured sequence of these operations to form actual devices and patterns can be made by either etching or lift off methods.
One of the most basic building blocks of MEMS processing is the ability to deposit thin films of materials that have different properties like insulators, semiconductors, conductors or special reactivity. The thin films can have a thickness anywhere from a few nanometers to several hundred micrometers. Films can subsequently be locally etched or lifted off to form patterns in the MEMS processes some of which are described below.
MEMS deposition technology can be classified into two groups called 1) depositions that happened because of a chemical reaction, such as chemical vapor deposition (CVD), electro deposition, epitaxy, and thermal oxidation; or 2) depositions that occur because of a physical reaction: such as physical vapor deposition (PVD) or casting. The chemical reaction processes exploit the creation or removal of solid materials directly from the surface by chemical reactions and gas and/or liquid interactions with the substrate material. The solid material is usually not the only product formed by the reaction. By-products can include gases, liquids or even other solids. The physical deposition processes have in common that the material deposited is physically moved onto the substrate. In other words, there is no chemical reaction which forms the material on the substrate. In the chemical reaction, a film or deposits can be made by electrodeposition or by thermal reaction of a gas with a hot substrate which are chemical reactions.
Lithography in the MEMS context is typically the transfer of a pattern to a photosensitive material by selective exposure to a radiation source such as light. Photosensitive materials are materials that experience a change in physical properties when exposed to a radiation source. If we selectively expose a photosensitive material to radiation (e.g. by masking some of the radiation), the pattern of the radiation on the materials is transferred to the photosensitive material exposed, as the properties of the exposed and unexposed regions differ. The washing of the unreacted materials leaves behind the patterned material in the desired pattern dictated by the mask. Subsequent depositions allow the layer to contact only the desired portions of the surface and subsequent removal of the photosensitive material allows the patterning of the deposited layer.
In order to form a functional MEMS structure on a substrate, it is necessary to etch the thin films previously deposited and/or the substrate itself. In general, there are two classes of etching processes: 1) wet etching where the material is dissolved when immersed in a chemical solution, or 2) dry etching where the material is sputtered or dissolved using reactive ions or a vapor phase etchant. As one skilled in the art will appreciate, advances in MEMS processing are ongoing and atomic layer deposition, plasma etching, and deep reactive ion etching and such techniques are constantly being advanced and developed to aid in the manufacture of tiny MEMS structures. There is a need within NASA and elsewhere in the gas detection community for low power gas sensors for analytical and safety applications on the ground and on board manned and unmanned vehicles and vessels. Critical gas analytes must be measured to ensure proper function of the on-board equipment and processes, and to ensure the safety of the crew on the ground and in flight. NASA applications include unique ground operations and remote travel. In such environments, power, size and weight are at a premium and resources are limited. Also, carrying spares, maintenance items and calibration equipment or consumables is undesirable. This means that the successful sensor must be stable and not require spare parts for calibrations for many years. Specifically, there is a need for an ultra-low power, high performance gas sensor platform capable of measuring He, H2 and other gases in air and process streams on the ground and in space that is stable for years. There are of course these benefits in many other commercial and consumer applications like cell phones.
It is the desire herein for the development of a unique approach to generate a MEMS sensor platform with wide-spread applicability with advanced analytical capability and significant commercial potential. As such, there is a need for apparatuses and new methods for microfabricating multi-dimensional nano-sensor platforms. Accordingly, improved apparatus and methods for using the same are desired. Since MEMS processing has the largest applicability and advantage for large applications and not all chemical sensors applications are large, MEMS is not typically applied to the development of many kinds of chemical sensors. Therefore to achieve commercial viability for the MEMS processes with many chemical sensors, it is advantageous to have many sensors capable of being built on the same MEMS platform made with common MEMS processes. In addition to the versatility of the individual MEMS structure, multiple structures on the same die will result in both redundancy for higher reliability and long lifetime as well as each area functionalized differently providing orthogonally responding devices on the same platform. While building tiny MEMS sensors can be achieved, many of the smallest structures can lack stability or corrode or degrade in performance rapidly especially when operated at elevated temperatures and in real environments. Accordingly, there is also a continuing need for a device that it is stable for long lifetimes and yet is still very small and low power in operation.